Roof racks for automobiles are available in numerous varieties, variously adapted to particular roof shapes or the types of load to be supported. A critical feature of all such racks is that they be attached to the roof in a secure manner which is not susceptible to loosening and disengagement when the vehicle encounters road surface deviations or sudden stops and starts. Securement to the roof is generally achieved by clamping elements which clip to the door frames on either side of the vehicle, pressing the rack pedestals down on the roof surface. Typical automobile roofs, of course, are made of thin sheet metal which does not support much force and are sometimes dented when clamping elements are tightened.
The risk of denting the roof as well as the risk of disengagement of the rack are due in part to the magnitudes and directions of the forces exerted by the pedestals and by the clips which secure the pedestals to the roof. The force exerted by each pedestal and its clip has a horizontal component and a vertical component. The horizontal component is borne primarily by the clip and is inwardly directed, i.e., toward the opposing clip, securing the rack to the roof by compressing the roof between the two clips. The vertical component compresses the pedestal down on the roof surface while pulling upward on the clip. The ratio of these components is important in maintaining a secure attachment. If the horizontal component is too great relative to the vertical component, the rack will have a tendency to slide along the vehicle roof longitudinally relative to the vehicle. If the vertical component is too great relative to the horizontal component, the risk of denting the roof is greatly increased. For both a minimal risk of roof damage and a minimal risk of sliding, the optimal ratio of vertical to horizontal force is 1:1.
Since many vehicles have roofs with curved edges, it is difficult to design a single pedestal and clip arrangement which will provide a secure yet damage-free attachment to the roof, and which can be adjusted to accommodate different angles and radii of curvature. It is particularly difficult to design a pedestal and clip combination which will maintain a constant ratio of the vertical to horizontal force components throughout the range of clamping, i.e., over the range of length adjustment of the clip relative to the pedestal. Pedestal and clip arrangements of the prior art in fact have ratios which range from about 1:1 to about 6:1 or 7:1. This severely limits both their effectiveness and their versatility.
The present invention solves this and other problems of the prior an by providing a pedestal clamping mechanism which maintains a substantially constant ratio of vertical to horizontal force component over its entire range of movement during the tightening of the clip and over the range of adjustability of the clip relative to the pedestal.